It is well known that the mechanical properties of synthetic organic fibers are strongly dependent upon the chain length of the molecules comprising them and their degree of orientation with respect to the fiber axis. If the molecular chain length falls below a certain level (which varies according to the type of material), the resulting chain ends and small molecules act as defects that substantially limit fiber tensile strength. It is therefore preferred in synthetic fiber production to extrude fibers from solutions or melts in which the number of low molecular weight molecules has been reduced as much as possible and that have the highest average chain length consistent with processibility. In general when the inherent viscosity of silk fibroin falls below a value of about 0.8 dL/g, the molecular weight has been reduced to the extent that low strength, brittle fibers are obtained on extrusion. The mechanical properties of a variety of silks and other fibers can be found in Kirk-Othmer Encyclopedia of Chemical Technology, 4th edition, volume 22, pages 160–161, herein incorporated by reference. Hence, the identification of new solvents that do not reduce the molecular weight of silk fibroin on a time scale that is useful for commercial production processes is of great utility for regenerated silk fiber production. The current invention enables preparation of fibers having useful tensile properties.
S. S. Raje, Rekha V. D and M. R. Mathur, Man-Made Textiles in India (April 1998), pp. 160–167, discloses the use of formic acid/water solutions with CaCl2 as a solvent for raw silk. Significant molecular weight loss of the silk fibroin is demonstrated by intrinsic viscosity reductions. C. Earland and D. J. Raven, Nature (4427, Sep. 4, 1954) p. 461, discloses the dissolution of silk in a solution of CaCl2 in formic acid containing 2% water. Significant reductions in intrinsic viscosity are noted in this article as well.
U.S. Pat. No. 5,252,285 discloses a process for spinning silk fibers after first dissolving silk fibroin in an aqueous salt solution, removing the salt from the solution, removing water to form the regenerated silk material, and then dissolving the silk material in hexafluoroisopropanol to form a fiber-spinnable solution. The two-step procedure is necessary because the aqueous silk solution is not useful for fiber spinning because of its high sensitivity to shear stress causing it to rapidly precipitate and block the spinneret capillaries during extrusion. As a consequence, the silk fibroin must be isolated from aqueous solution and redissolved in solvents such as hexafluoroisopropanol or mixtures of formic acid with lithium salts so that extrusion can be carried out without shear induced precipitation.
Although hexafluoroisopropanol provides regenerated silk fibers with excellent mechanical properties it is not an attractive spinning solvent because of its high toxicity and cost. Likewise, although mixtures of formic acid with lithium salts provide shear stable spinning solutions the as-spun fiber mechanical properties obtained are not useful for textile and apparel applications and require further downstream processing to develop useful mechanical properties.